The present disclosure relates generally to switching mode power supplies.
Switching mode power supplies (SMPS) typically utilize a power switch to control the current through an inductive device in order to regulate a current output or a voltage output. In comparison with other kinds of power supplies, SMPS are commonly compact and power efficient, so as being popular nowadays.
One kind of SMPS operates in quasi-resonance (QR) mode and is referred to QR converters. The power switch in a QR converter is switched from an OFF state (performing an OFF circuit) to an ON state (performing a short circuit) substantially at the moment when the voltage drop across the power switch is at a minimum, so the switching loss might be minimized, theoretically. Observation has proofed that the power conversion efficiency of a QR converter is really excellent especially when it supplies power to a heavy load.
FIG. 1 demonstrates a QR converter 10 in the art, where a transformer, an inductive device, has a primary winding PRM, a secondary winding SEC and an auxiliary winding AUX, all inductively coupled to each other. The QR converter 10 is powered by input voltage VIN, to supply power, in the form of output voltage VOUT and output current IOUT, to an output load 24. QR converter 10 provides pulse-width-modulation (PWM) signal VGATE at driving node GATE to periodically turn ON and OFF a power switch 34. Via the voltage division provided from resistors 28 and 30, QR converter 10 further monitors voltage drop VAUX across the auxiliary winding AUX. FIG. 2 illustrates the waveforms of PWM signal VGATE and voltage drop VAUX. Shown in FIG. 2, two consecutive rising edges of PWM signal VGATE define one switching cycle, whose duration is referred to as a cycle time TCYC consisting of an ON time TON and an OFF time TOFF, where the ON time TON and the OFF time TOFF are the durations when the power switch 34 is kept as being ON and OFF, respectively. FIG. 2 also demonstrates that the ON time TON is also the pulse width of the PWM signal VGATE. Demonstrated in FIG. 2, about the middle of the OFF time TOFF, the voltage drop VAUX starts oscillating because of the power depletion of the transformer, and signal valleys VL1 and VL2 are therefore generated. QR controller 26 ends a cycle time TCYC or an OFF time TOFF at the moment when signal valley VL2 substantially occurs as demonstrated in FIG. 2. This kind of method to end a cycle time TCYC in a signal valley is known and referred to as valley switching.
QR converter 10 has, at a compensation node COMP, a compensation signal VCOMP, controlled by operational amplifier (OP) 20, in response to the difference between the output voltage VOUT and a target voltage VTAR. The compensation signal VCOMP in the QR converter 10 controls both the ON time TON and a block time TBLOCK, where the next switching cycle is not allowed to start until the block time TBLOCK ends. The block time TBLOCK prevents a switching frequency fCYC, the reciprocal of a cycle time TCYC, from being over high. An over-high switching frequency fCYC probably lowers the power conversion due to the more power loss in charging and discharging the driving node GATE. The block time TBLOCK equivalently defines a maximum switching frequency fCYC-MAX, which is the reciprocal of the block time TBLOCK.
QR converter 10 usually encounters two issues.
The first issue is the hardship to solve electromagnetic interference (EMI). For a constant output load 24, the compensation signal VCOMP could be a constant, and the power switch 34 is turned on in a certain signal valley to conclude a cycle time TCYC, implying a constant switching frequency fCYC and intensive EMI, normally unacceptable in the art. A known solution for this EMI issue is to intentionally disturb the compensation signal VCOMP. The feedback loop provided by the operational amplifier 20 in FIG. 1, however, tends to cancel any disturbance introduced to the compensation signal VCOMP. Therefore, this solution hardly helps the EMI issue.
Another issue is the occurrence of intolerable audible noise. In some conditions with a certain output load 24, the compensation signal VCOMP spontaneously vibrates, and QR controller 26 performs valley switching not constantly in a certain signal valley, but alternatively in two adjacent signal valleys. In other words, due to the vibration of the compensation signal VCOMP, valley switching might be first in a certain signal valley for several switching cycles, then followed by shifting to be in an adjacent signal valley for a while, and then further followed by shifting back to be in the certain signal valley for a while, and so forth. This instability in valley switching could result in audible noise, which is normally intolerable in the market, especially for the applications targeting to a quiet environment.